Digital subscriber carrier system including drop terminals and bypass and loopback means and alarm indication means

Abstract

A multi-channel digital subscriber carrier system for serving a multiplicity of subscribers one or more of whom may be located at points between the central office and the subscriber (furthest remote) terminal. One or more drop terminals are provided to serve these inbetween subscribers. Each terminal has a receiver section for recovering the clock signal generated by a master oscillator located in the central office. The receiver section also synchronizes on the received signal and generates signals indicating times of occurrence of the channels in the received signal. The transmitter section of each terminal uses the above signals and the recovered clock to generate a digital signal wherein messages from the subscribers at the terminal are inserted in those channels associated with the subscribers for the messages received from the office for the subscribers. Each terminal has associated therewith a circuit for bypassing the terminal upon the occurrence of a terminal fault condition. When the drop terminals are connected to the inward cable pair each terminal has associated therewith a circuit for looping back the system at the terminal upon the occurrence of a cable fault condition. The loopback circuit operates automatically and independently of the central office. An alarm mechanism is provided to allow a serviceman to easily locate the fault condition.

What is claimed is: 
     
       1. A drop terminal for serving at least one subscriber of a multi-channel digital subscriber carrier system, said system including a central office for transmitting over a first transmission means messages to said system subscribers and for receiving over a second transmission means messages from said subscribers and an oscillator for generating a clock signal having a predetermined frequency, said terminal being connected to one of said transmission means at a location external to said central office said terminal receiving from said transmission means a periodic first digital message signal having a transmission frequency equal to said clock frequency and a multiplicity of channel times each associated with a respective one of said subscribers said channels occurring in said signal in a predetermined time sequence at least one of said channel times having an encoded message signal for said at least one drop terminal subscriber, said terminal transmitting on said transmission means a periodic second digital message signal having said transmission frequency and said multiplicity of channels times occurring in said predetermined time sequence said channel associated with said at least one drop terminal subscriber having an encoded message from said subscriber said second signal having a fixed time relationship to said first signal said terminal comprising: (a) means for receiving said first signal including: (i) means responsive to said first signal for recovering said clock signal therefrom;   (ii) means responsive to said recovered clock signal and said first signal for identifying said predetermined time of occurrence of each of said channels and for synchronizing on said period of said first signal said means generating two signals one of which identifies said channel time of occurrence and the other of which is indicative of said synchronization;   (iii) means responsive to said first signal for decoding all of said messages encoded therein; and   (iv) means responsive to said decoded message signals, said channel identifying signal and said synchronizing signal for providing said at least one drop terminal subscriber with its message; and     (b) means for transmitting said second signal including: (i) means responsive to said recovered clock, said channel identifying signal, said synchronizing signal and said message from said at least one subscriber at said terminal for inserting said message in said channel time associated with said subscriber and for encoding said inserted message said means generating a periodic digital signal having said transmission frequency and said multiplicity of channels occurring in said predetermined time sequence; and   (ii) means responsive to said signal generated by said inserting and encoding means and said first signal for digitally multiplexing said encoded signal and said first signal to thereby generate said second digital signal.     
     
     
       2. A multi-channel digital subscriber carrier system for serving a multiplicity of subscribers remote from a central office said system comprising: (a) a first transmission means connected to said central office, said office transmitting messages to said system subscribers over said means;   (b) a second transmission means connected to said central office, said office receiving messages from said system subscribers over said means;   (c) a subscriber terminal for serving at least one of said subscribers;   (d) oscillator means located in said central office for generating a clock signal having a predetermined frequency; and   (e) a drop terminal for serving at least a different one of said subscribers said terminal being connected to one of said transmission means at a location between said office and said subscriber terminal, said drop terminal receiving from said transmission means a periodic first digital message signal having a transmission frequency equal to said clock frequency and a multiplicity of channel times each associated with a respective one of said system subscribers said channels occurring in said signal in a predetermined time sequence at least one of said channel times having an encoded message for said at least one different drop terminal subscriber said terminal transmitting on said transmission means a periodic second digital message signal having said transmission frequency and said multiplicity of channel times occurring in said time sequence said channel associated with said at least one different drop terminal subscriber having an encoded message from said subscriber said second signal having a fixed time relationship to said first signal said drop terminal having means for receiving means said first signal and means for transmitting said second signal said receiving means including: (i) means responsive to said first signal for recovering said clock signal therefrom;   (ii) means responsive to said recovered clock signal and said first signal for identifying said predetermined time of occurrence of each of said channels and for synchronizing on said period of said first signal said means generating two signals one of which identifies said channel time of occurrence and the other of which is indicative of said synchronization;   (iii) means responsive to said first signal for decoding all of said messages encoded therein; and   (iv) means responsive to said decoded message signals, said channel identifying signal and said synchronizing signal for providing said at least one drop terminal subscriber with its message; and     (f) said transmitting means including: (i) means responsive to said recovered clock, said channel identifying signal, said synchronizing signal and said message from said at least one subscriber at said terminal for inserting said message in said channel time associated with said subscriber and for encoding said inserted message said means generating a periodic digital signal having same transmission frequency and said multiplicity of channels occurring in said predetermined time sequence; and   (ii) means responsive to said signal generated by said inserting and encoding means and said first signal for digitally multiplexing said encoded signal and said first signal to thereby generate said second digital signal.     
     
     
       3. The system of claim 2 wherein said drop terminal is connected to said first transmission means. 
     
     
       4. The system of claim 3 wherein said subscriber terminal has a receiver means and a transmitter means said receiver means being connected to said first transmission means for receiving said second signal transmitted by said drop terminal, said transmitter means being connected to second transmission means and generating a third periodic digital message signal to said central office. 
     
     
       5. The system of claim 4 wherein said first signal received by said drop terminal includes in at least another one channel time associated with said subscriber terminal subscriber an encoded message for said subscriber said receiver means in said subscriber terminal including: (a) means responsive to said second signal for recovering said clock signal therefrom;   (b) means responsive to said recovered clock signal and said second signal for identifying said predetermined time of occurrence of each of said channels and for synchronizing on said period of said second signal said means generating two signals one of which identifies said channel time of occurrence and the other of which is indicative of said synchronization;   (c) means responsive to said second signal for decoding all of said messages encoded therein; and   (d) means responsive to said decoded message signals, said channel identifying signal and said synchronizing signal for providing said at least one subscriber terminal subscriber with its message.   
     
     
       6. The system of claim 5 wherein said transmitter means of said subscriber terminal includes: (a) means responsive to said recovered clock, said channel identifying signal, said synchronizing signal and said message from said at least one subscriber at said terminal for inserting said message in said channel time associated with said subscriber and for encoding said inserted message said means generating a periodic digital signal having said transmission frequency and said multiplicity of channels occurring in said predetermined time sequence; and   (b) means responsive to said signal generated by said inserting and encoding means and said second signal for digitally multiplexing said encoded signal and said second signal to thereby generate said third digital signal.   
     
     
       7. The system of claim 2 wherein said drop terminal is connected to said second transmission means. 
     
     
       8. The system of claim 7 wherein said subscriber terminal has a receiver means and a transmitter means said receiver means being connected to said first transmission means to receive a third periodic digital message signal from said central office said third signal having a transmission frequency equal to said clock frequency and a multiplicity of channel times each associated with a respective one of said system subscribers said channels occurring in said third signal in a predetermined time sequence at least one of said channel times having an encoded message signal for said at least one different drop terminal subscriber and at least another one of said channel times having an encoded message signal for said at least one subscriber served by said subscriber terminal, said transmitter means being connected to second transmission means and generating said first periodic digital message signal to said drop terminal. 
     
     
       9. The system of claim 8 wherein said receiver means in said subscriber terminal includes: (a) means responsive to said third signal for recovering said clock signal therefrom;   (b) means responsive to said recovered clock signal and said third signal for identifying said predetermined time of occurrence of each of said channels and for synchronizing on said period of said third signal said means generating two signals one of which identifies said channel time of occurrence and the other of which is indicative of said synchronization;   (c) means responsive to said third signal for decoding all of said messages encoded therein; and   (d) means responsive to said decoded message signals, said channel identifying signal and said synchronizing signal for providing said at least one subscriber terminal subscriber with its message.   
     
     
       10. The system of claim 5 wherein said transmitter means of said subscriber terminal includes: (a) means responsive to said recovered clock, said channel identifying signal, said synchronizing signal and said message from said at least one subscriber at said terminal for inserting said message in said channel time associated with said subscriber and for encoding said inserted message said means generating a periodic digital signal having said transmission frequency and said multiplicity of channels occurring in said predetermined time sequence; and   (b) means responsive to said signal generated by said inserting and encoding means and said first signal for digitally multiplexing said encoded signal and said first signal to thereby generate said second digital signal.   
     
     
       11. A multi-channel digital subscriber carrier system for serving a multiplicity of subscribers remote from a central office said system comprising: (a) an outward transmission line connected to said central office, said office transmitting messages to said system subscribers over said line;   (b) an inward transmission line connected to said central office, said office receiving messages from said system subscribers over said line;   (c) a subscriber terminal for serving at least one of said subscribers;   (d) oscillator means located in said central office for generating a clock signal having a predetermined frequency; and   (e) a drop terminal for serviing at least a different one of said subscribers said terminal being connected to said outward transmission line at a location between said office and said subscriber terminal, said drop terminal receiving from said transmission line a periodic first digital message signal having a transmission frequency equal to said clock frequency and a multiplicity of channel times each associated with a respective one of said system subscribers said channels occurring in said signal in a predetermined time sequence at least one of said channel times having an encoded message for said at least one different drop terminal subscriber said terminal transmitting on said outward transmission line a periodic second digital message signal having said transmission frequency and said multiplicity of channel times occurring in said time sequence said channel associated with said at least one different drop terminal subscriber having an encoded message from said subscriber said second signal having a fixed time relationship to said first signal said drop terminal having means for receiving means said first signal and means for transmitting said second signal said receiving means including: (i) means responsive to said first signal for recovering said clock signal therefrom;   (ii) means responsive to said recovered clock signal and said first signal for identifying said predetermined time of occurrence of each of said channels and for synchronizing on said period of said first signal said means generating two signals one of which identifies said channel time of occurrence and the other of which is indicative of said synchronization;   (iii) means responsive to said first signal for decoding all of said messages encoded therein; and   (iv) means responsive to said decoded message signals, said channel identifying signal and said synchronizing signal for providing said at least one drop terminal subscriber with its message; and     (f) said transmitting means including: (i) means responsive to said recovered clock, said channel identifying signal, said synchronizing signal and said message from said at least one subscriber at said terminal for inserting said message in said channel time associated with said subscriber and for encoding said inserted message said means generating a periodic digital signal having said transmission frequency and said multiplicity of channels occurring in said predetermined time sequence; and   (ii) means responsive to said signal generated by said inserting and encoding means and said first signal for digitally multiplexing said encoded signal and said first signal to thereby generate said second digital signal.     
     
     
       12. A multi-channel digital subscriber carrier system for serving a multiplicity of subscribers remote from a central office said system comprising: (a) an outward transmission line connected to said central office, said office transmitting messages to said system subscribers over said line;   (b) an inward transmission line connected to said central office, said office receiving messages from said system subscribers over said line;   (c) a subscriber terminal for serving at least one of said subscribers;   (d) oscillator means located in said central office for generating a clock signal having a predetermined frequency; and   (e) a drop terminal for serving at least a different one of said subscribers said terminal being connected to said inward transmission line at a location between said office and said subscriber terminal, said drop terminal receiving from said transmission line a periodic first digital message signal having a transmission frequency equal to said clock frequency and a multiplicity of channel times each associated with a respective one of said system subscribers said channels occurring in said signal in a predetermined time sequence at least one of said channel times having an encoded message for said at least one different drop terminal subscriber said terminal transmitting on said onward transmission line a periodic second digital message signal having said transmission frequency and said multiplicity of channel times occurring in said time sequence said channel associated with said at least one different drop terminal subscriber having an encoded message from said subscriber said second signal having a fixed time relationship to said first signal said drop terminal having means for receiving said first signal and means for transmitting said second signal said receiving means including: (i) means responsive to said first signal for recovering said clock signal therefrom;   (ii) means responsive to said recovered clock signal and said first signal for identifying said predetermined time of occurrence of each of said channels and for synchronizing on said period of said first signal said means generating two signals one of which identifies said channel time of occurrence and the other of which is indicative of said synchronization;   (iii) means responsive to said first signal for decoding all of said messages encoded therein; and   (iv) means responsive to said decoded message signals, said channel identifying signal and said synchronizing signal for providing said at least one drop terminal subscriber with its message; and     (f) said transmitting means including: (i) means responsive to said recovered clock, said channel identifying signal, said synchronizing signal and said message from said at least one subscriber at said terminal for inserting said message in said channel time associated with said subscriber and for encoding said inserted message said means generating a periodic digital signal having said transmission frequency and said multiplicity of channels occurring in said predetermined time sequence; and   (ii) means responsive to said signal generated by said inserting and encoding means and said first signal for digitally multiplexing said encoded signal and said first signal to thereby generate said second digital signal.     
     
     
       13. The system of claim 12 wherein said drop terminal has an input connected to said receiver means and said system includes loopback means located between said subscriber terminal and said input said loopback means normally connecting said input to said inward line and upon the occurrence of at least one of a predetermined number of fault conditions in said first digital signal said loopback means disconnecting said normal connection and connecting said input to said outward line. 
     
     
       14. The system of claim 13 wherein said loopback means includes: (a) means responsive to said first digital signal for determining the occurrence of at least one of said predetermined number of said fault conditions and generating a signal upon said occurrence; and   (b) switching means connected between said inward and outward lines and said input responsive to said fault occurrence signal for providing said connection between said input and said outward line said switching means providing said normal connection when said fault occurrence means does not generate a signal.   
     
     
       15. The system of claim 14 wherein said subscriber terminal has an input connected to said outward line and an output connected to said inward line and said terminal includes receiver means connected between said input and said at least one subscriber and transmitter means connected between said at least one subscriber and said output and bypass means responsive to the occurrence of at least one of a predetermined number of terminal fault conditions for disconnecting said output from said transmitter means and providing a direct electrical connection between said input and said output. 
     
     
       16. The system of claim 13 wherein said drop terminal has an output connected to both said inward line and said transmitter means and includes bypass means for disconnecting said transmitter means from said output upon the occurrence of at least one of a predetermined number of drop terminal fault conditions and providing a direct electrical connection between said input and said output. 
     
     
       17. The system of claim 14 wherein said drop terminal has an output connected to both said inward line and said transmitter means and includes bypass means for disconnecting said transmitter means from said output upon the occurrence of at least one of a predetermined number of drop terminal fault conditions and providing a direct electrical connection between said input and said output. 
     
     
       18. A loopback circuit for a serially looped multichannel digital subscriber carrier system having at least two remote terminals each having an input and an output and at least one system subscriber connected thereto wherein a central office is connected by an outward line directly to said input of the furthest remote of said remote terminals and is connected by an inward line with each of said remote terminals, said central office transmitting a signal having a clock and messages encoded therein for each of said subscribers connected to each of said terminals said loopback circuit located between said furthest remote terminal output and said input of said other terminal, said circuit comprising: (a) an input always connected to said inward line at said location for receiving said encoded messages signal on said inward line;   (b) first means responsive to said signal for recovering said clock therefrom the failure of said means to recover said clock within a predetermined period of time being indicative of an error in said signal;   (c) second means responsive to said recovered clock and said signal for determining the occurrence in said signal of at least one of a predetermined number of errors other than said clock recovery error has occurred in said signal said means generating a signal indicative of the occurrence of each of said errors, each of said errors having a predetermined time associated therewith;   (d) timing means responsive to said recovered clock and said error indicative signals generated by said second means said timing means having said predetermined time associated with said clock recovery and said predetermined times associated with a respective one of each of said predetermined other errors said timing means generating an error signal whenever any one of any of said errors is present by no later than the end of the associated one of said predetermined times; and   (e) switching means having a control connected to receive said timing means error signal said switching means connected between said inward and outward lines and said other input said switching means normally connecting said other input to said inward line in the absence of said timing means error signal said switching means being operated to connect said other input to said outward line when said timing means generates said error signal to thereby disconnect said normal connection and loopback said system at said location.   
     
     
       19. The circuit of claim 18 wherein said second means includes means responsive to said recovered clock and said signal for synchronizing on said signal and one of said predetermined other errors is a failure to accomplish said synchronizing. 
     
     
       20. The circuit of claim 18 wherein said signal is bipolar in nature and said second means includes means responsive to said recovered clock and said signal for determining if said signal has bipolar violations and one of said predetermined other errors is a failure of said signal to be bipolar. 
     
     
       21. The loopback circuit of claim 20 wherein said signal has channel times slots equal in number to the number of channels in said system each of said slots being associated with a respective one of said subscribers, the time for serially transmitting all of said channel time slots being a frame said bipolar violation determining means including means for detecting a predetermined number of bipolar violations occurring in each frame for a predetermined number of said frames, said detecting means generating a bipolar violation signal when said number of violations in each frame is detected for said predetermined number of frames. 
     
     
       22. A bypass circuit adapted for use in the remote terminal of a digital subscriber carrier system, said terminal serving one or more subscribers connected thereto and having an input for receiving a first digital signal having at least encoded messages for said subscribers and an output for transmitting a second digital signal connected thereto said second signal having at least encoded messages from said subscribers, said circuit comprising: (a) means connected to said terminal and responsive to said first digital signal for detecting the occurrence of an error therein and generating a receive error signal when the occurrence of said error is detected;   (b) switching means connected between said input and said output said switching means being kept energized in a first position in the absence of said receive error signal said switching means not providing in said first position a direct electrical connection between said input and said output; and   (c) means for connecting said receive error signal to said switching means said switching means deenergizing to assume a second position in response to the occurrence of said error signal said second position disconnecting said second signal from said output and providing a direct electrical connection between said input and said output said circuit maintaining said connection of said means responsive to said first digital signal to said terminal whereby said terminal is bypassed.   
     
     
       23. The circuit of claim 22 wherein said switching means includes means responsive to a loss of power at said terminal for causing said switching means to change from said first position to said second position thereby bypassing said terminal when said loss of power occurs. 
     
     
       24. The circuit of claim 22 further including means connected to said terminal and responsive to said second digital signal for detecting the occurrence of an error therein and generating a transmit error signal when the occurrence of said error is detected said transmit error signal being connected to said switching means said switching means responsive thereto to assume said second position. 
     
     
       25. The circuit of claim 24 wherein each of said means for detecting an error in said first and second digital signals also generates a signal when the associated one of said errors terminates said switching means responsive to said terminating signal for returning to said first position. 
     
     
       26. A multichannel digital subscriber carrier system comprising: (a) a central office;   (b) a transmission line extending outward from said central office for carrying a message signal from said office to said system subscribers;   (c) a transmission line extending inward to said central office for carrying a message signal from said system subscribers to said office;   (d) a subscriber terminal having an input connected in said outward line and an output connected to said inward line said terminal serving at least one subscriber of said system said terminal receiving said office message signal from said outward line and recovering therefrom messages for said at least one subscriber served by said terminal said terminal transmitting on said inward line a first digital message signal wherein messages from said at least one subscriber to said central office have been substituted for said recovered messages;   (e) at least one drop terminal having an input and an output connected to said inward line at a location between said central office and said subscriber terminal for serving at least one other subscriber of said system said terminal receiving at said input said first message signal and recovering therefrom messages from said central office for said at least one other subscriber served by said terminal and generating at said output a second message signal wherein messages from said at least one other subscriber to said central office have replaced said recovered message signal; each of said terminals including bypass means connected between said input and said output for effecting a direct electrical connection between said input and said output upon the occurrence of at least one of a predetermined number of fault conditions at said terminal; and   (f) loopback means having an input connected to said inward line to receive said first signal and first and second outputs said first output being connected to said drop terminal input, said second output being connected to said outward line said loopback means input being normally connected to said first output said loopback means disconnecting said normal connection and connecting said loopback means input to said second output upon the occurrence of at least one of a predetermined number of transmission line fault conditions at said loopback means.   
     
     
       27. In an N channel digital subscriber carrier system having a central office and at least one remote terminal for serving at least one of said subscribers connected thereto by a transmission medium and in which information signals are transmitted on said medium in a first predetermined format which is divided into a predetermined repetitive number of frames each of which is divided into N channels each having a predetermined number of bits, each of said N channels being associated with a respective one of said subscribers, an alarm system comprising: (a) central office alarm means including: (i) means for identifying when said signal is in a second predetermined format which is related to said first predetermined format a regularly recurring bit in a predetermined one of said frames for at least said channel associated with said at least one subscriber and inserting therein a preselected value; and   (ii) means responsive to said information signal when said signal is in said second format with said preselected bit value for transmitting said signal in said first format in serial fashion on said medium;     (b) remote terminal alarm means including: (i) means responsive to said signal in said medium for converting said signal to said second format;   (ii) conditioning means having an output responsive to said signal in said second format for changing said value in said regularly recurring bit to a value opposite to said value inserted in said central office alarm means said means operating not to change said value upon the occurrence of at least one of a predetermined fault condition in said system; and   (iii) means connected to said conditioning means output for transmitting said signal in said first format in serial fashion; and     (c) means included in said central office means responsive to said signal in said first format received from said medium for converting said signal to said second format and in response thereto for comparing the value of said received bit to the value of said second format transmitted bit and for generating an alarm signal when said values are the same.   
     
     
       28. The alarm system of claim 27 wherein said central office alarm means further includes means responsive to each occurrence of said alarm signal for inserting in a preselected bit of said channel associated with said at least one subscriber for one frame of said information signal when said signal is in said first format, a preselected value. 
     
     
       29. The alarm system of claim 28 wherein said remote terminal alarm means further includes means responsive to said information signal in said medium for determining the number of consecutive frames in which said predetermined bit of said channel corresponding to said at least one subscriber has said preselected value and generating a central office alarm signal when said number of consecutive frames exceeds a predetermined number.